Direct sound, reflections, and reverberation

A listener in a room hears both the direct sound from the speaker and many reflections from the walls, ceiling, floor, and other surfaces. The greater the proportion of reflected energy relative to direct energy, the more the room influences the perceived sound.

The first reflections arrive shortly after the direct signal. They can affect timbre, source localization, and clarity. Later reflections begin to create a reverberant field.

In practice, this means that two identical systems set up in two rooms can have very different:

• sound,
• clarity of vocals,
• understanding of speech,
• bass distribution,
• susceptibility to feedback.

A good design should therefore increase the proportion of useful direct sound and limit unnecessary excitation of the room. 

What does reverberation time mean?

Reverberation time describes how long acoustic energy remains in a room after the source has stopped. In practice, the parameter RT60 is often used, which is the time needed for the reverberant field level to drop by 60 dB.

There is no single ideal value for every space. A room intended for speeches usually requires a shorter and more controlled reverberation than a space designed for specific musical applications.

A long reverberation time causes subsequent sounds to start overlapping. In music, this can reduce rhythmic precision, and in speech, it can cause syllables to blur.

It is also important that the reverberation time can vary in different frequency ranges. A room can be relatively dry in the upper range while simultaneously retaining energy for a long time in low frequencies. 

Why does an empty room behave differently than a full one?

The audience is an acoustic element of the room. People, clothing, upholstered chairs, and other soft surfaces absorb some of the energy, especially in the mid and high frequency ranges.

Therefore, an empty room during a rehearsal may have:

• longer reverberation,
• more distinct reflections,
• brighter and more aggressive sound,
• a different tonal balance.

When the audience enters, some of the energy is absorbed. The reverberation may shorten, and the character of the room may change.

This does not mean that a full room automatically sounds good. Problems related to geometry, low-frequency modes, or poor PA setup still remain.

Practically, this means that tuning an empty room should be approached with some margin. The engineer should anticipate that the space will behave differently once filled with an audience.

How do surface materials affect sound?

Different materials reflect and absorb sound to varying degrees and across different frequency ranges.

Hard surfaces, such as:

• concrete,
• glass,
• tiles,
• hard stone,

usually reflect a large portion of energy across a wide range.

Soft materials, such as:

• curtains,
• thick carpets,
• upholstered chairs,
• porous materials,

can absorb a larger portion of energy, especially in the mid and high frequencies.

However, this does not mean that a curtain will solve every acoustic problem. Low frequencies have very long wavelengths and require different control methods.

In practice, a room with a large number of glass surfaces may require a system with significantly better directional control than a room of similar dimensions but with a lot of absorbing materials.

Why does high reverberation reduce speech intelligibility?

Speech consists of rapidly following syllables and sounds. If earlier elements of the speech remain in the room for a long time, they begin to mask the subsequent ones.

The listener may then hear a sufficient level, but have a problem understanding the content.

This is particularly important during:

• conferences,
• presentations,
• panel discussions,
• events with many open microphones.

Increasing the PA level does not always help. If the system simultaneously sends even more energy to the walls and ceiling, the reverberation field also becomes stronger.

Therefore the sound system for conferences aimed at high speech intelligibility should primarily ensure a good ratio of direct sound to reflected.

In a difficult room, a more directional system working closer to the audience may be more effective than one very powerful set placed far away.

How does the directionality of the PA help in difficult acoustics?

Directionality allows deciding where the energy of the system goes. In a room, this is particularly important because sound sent beyond the audience does not disappear — it reflects and returns to the listeners.

If the set has too wide coverage, part of the energy may hit:

• side walls,
• the ceiling,
• the back wall,
• areas, that do not require amplification.

The more unnecessary energy hits reflective surfaces, the greater the contribution of the reverberation field.

Therefore, in a difficult room, often more important than the maximum SPL is how precisely the system can cover the audience.

This applies to both point source systems and line arrays. The name of the technology itself does not guarantee proper directionality.

The topic is further developed in the material Linear system or classic PA sets – what to choose for the event?

When is a distributed system more advantageous?

A distributed system is beneficial when a single source would have to operate at a very high level or send energy over a long distance through an acoustically challenging room.

Instead of one system at the stage, several smaller sources placed closer to the audience can be used.

This provides several benefits:

• a shorter distance between the speaker and the audience,
• a lower required SPL of a single source,
• a greater proportion of direct sound,
• the ability to limit the excitation of distant surfaces.

However, such a solution requires proper time alignment.

If additional systems play without delay control, they can cause echo, interference, or loss of sound localization relative to the stage.

In long halls and conference rooms, a distributed system can therefore be significantly more effective than trying to "punch through" the reverberation with one very powerful PA.

Can EQ correction fix poor acoustics?

EQ can improve some tonal issues, but it cannot change the geometry of the room or shorten the reverberation time. This is one of the most important distinctions when tuning a system.

What EQ can improve

Correction can help:

• limit overly emphasized frequency bands,
• equalize the tonal character of the system,
• correct the part of the influence of speaker placement,
• reduce resonances visible in a specific listening area.

If the system is too bright, dark, or has a distinct tonal imbalance resulting from its configuration, EQ is the right tool.

What EQ cannot fix

EQ will not remove physical reflections from the back wall.

It will also not shorten the reverb time or change the fact that the speaker sends a large amount of energy to the ceiling.

It will also not fix large level differences resulting from distance or poorly set delay systems.

Attempting to aggressively correct spatial issues may further degrade the sound in other areas of the room.

Therefore, the order should be as follows: proper system geometry, appropriate directivity, correct setup, and only then EQ and DSP.

How are acoustic measurements used?

Measurements allow separating subjective impressions from the actual properties of the system and the room.

Depending on the goal, one can analyze, among other things:

• frequency response,
• impulse,
• time relationships between sources,
• time energy decay,
• differences between audience points.

During PA tuning, measurements allow checking whether the main system, subwoofers, front fills, and delays are working according to the design.

It is worth measuring in several representative locations. Correction made solely based on one microphone standing at FOH may not represent the behavior of the entire room.

If there is a deep null in one place resulting from geometry and reflections, trying to boost it with a large correction may create an excess of energy in other parts of the audience.

Measurements are therefore a tool for decision-making, not a way to automatically "straighten" every room.

Acoustic adaptation or changing the PA system?

Adaptation and the proper PA system address different parts of the same problem.

Acoustic adaptation changes the behavior of the room. It can limit:

• undesired reflections,
• excessive reverberation time,
• some resonances,
• excessive energy in specific areas.

Changing the PA system does not modify the room itself, but it can limit the amount of energy that excites problematic surfaces.

In a permanent venue, the best results often come from combining both approaches.

For a temporary event, it is not always possible to interfere with the architecture, which is why the following become more important:

• directionality,
• placement of sources,
• mounting height,
• use of distributed systems.

In an exceptionally challenging venue, even the best PA cannot replace the appropriate adaptation. However, it can significantly reduce the scale of the problem.

What to check during the site visit?

The site visit should serve to gather the data needed for the project, not just to check where speakers can be placed.

It is essential to assess primarily:

• the length, width, and height of the room,
• the location of the stage,
• layout of the audience,
• materials of the walls and ceiling,
• large glazing,
• balconies and mezzanines,
• columns and other obstacles,
• possibilities for suspending the system,
• location of FOH,
• potential locations for fills and delays,
• noise from HVAC systems and other devices.

It is also good to listen to the room itself: applause or speech without amplification can already reveal strong reflections or a very long reverb.

If the venue is complex, analyzing the acoustics of the space and the selection of sound reinforcement before the event allows for a configuration to be prepared much more effectively than trying to solve all problems during installation.

For larger projects, the design can additionally be based on the layout of the venue and system prediction, and then verified with measurements after installation.

Summary

The acoustics of the venue directly affect how the PA system should be selected and set up. The same setup can work very well in one venue and significantly worse in another if the geometry, reverberation time, finishing materials, and audience layout change.

The most important task is to control the ratio of direct sound to reflected sound. This is primarily achieved through proper directionality, placement, and height of the system, and in large spaces also by using distributed sources.

EQ and DSP are necessary tools, but they will not replace proper geometry or acoustic adaptation. Therefore, the selection of sound systems should start with an analysis of the room, not just from the parameters of the speakers.

Frequently Asked Questions